A number of patents have been filed for safety devices for power tools that can lock the power switch so that the power tool is rendered inoperable. Most of these involve a physical barrier to turning on the power tool, but generally do not involve an actual lock that can prevent the power tool from being operated without unlocking the lock.
The U.S. Pat. Nos. 4,166,202 and 4,389,550, both by Reiter, are examples of safety switches that do use a lock to prevent the operation of a power tool. Patent '202 describes a system that can lock a power switch using a key. Once locked, the power switch cannot turn on the power tool, but an emergency button can turn the power tool off if it is already on. Patent '550 also is a similar system that uses a key lock to prevent the operation of the power switch. Patent '550 also has an emergency button that can turn off the power tool even when the switch is locked. The '550 also can be placed in the cord of the power tool to prevent operation. In both patents, the key lock is integrated into the housing for the switch and the housing is required to have an emergency off button.
While the '202 and '550 are useful, they are limited by the complex parts of each switch and housing, by the required emergency off button, and by the specific integrated lock. Both patents require that the lock is a key lock that operates by rotating a cam out of the path of the power switch. In fact, in both patents, the lock only works if it can block the path of the switch. The lock does not actually secure the switch or the toggle for the switch.